1. Field of the Invention
The present invention generally relates to a paper storage device and more particularly to the paper storage device used in an image forming apparatus such as, for example, an electrophotographic copying machine or a laser printer for accommodating, in a stacked fashion papers which are successively discharged from the image forming apparatus.
2. Description of the Prior Art
A laser printer is usually used for making a large number of copies from a single document at one time and, therefore various paper storage devices have been proposed such as having a tray of relatively large capacity or stackers. In general, the paper storage device of the type used in association with the laser printer comprises at least one paper support tray supported for stepwise movement between lowered and elevated positions. The paper support tray is generally held at the elevated position and is stepwise moved down to the lowered position according to the payload on the paper support tray, that is, the number or weight of the papers discharged from the laser printer onto the paper support tray. It is necessary to descend the paper support tray according to the payload imposed thereon for maintaining the distance, that is `drop height` between a nip of a pair of discharge rollers and a surface of papers stacked on the paper support tray constantly in order to achieve good disposition of paper discharged from the laser printer onto the paper support tray.
If an operator of the laser printer removes a portion of the stack of the papers from the paper support tray while the laser printer is in process of producing copies, the level of the uppermost one of the papers remaining on the paper support tray lowers and, hence, the drop height referred to above becomes larger. Once this occurs, a problem associated with a loss of orderly disposition of the papers on the paper support tray tends to occur.
In view of the foregoing, the prior art is such that the use of either upper and lower limit switches for detecting the arrival of the paper support tray at elevated and lowered positions, respectively, or a detector switch for detecting the surface of the uppermost one of the stack of the papers on the paper support tray, has been made to control the movement of the paper support tray in dependence on the state of one of those upper and lower limit switches or the state of the detector switch. See the Japanese Laid-open Patent Publication No. 00-137764, published in 1985, and U.S. Pat. No. 4,959,685.
However, according to any one of those prior art systems, when a drive means is driven always according to a change in drop height above the uppermost one of the papers, stacked on the paper support tray, to move the paper support tray between the elevated and lowered positions, they have problems associated with durability noises and others and, therefore, require the use of a structure for monitoring the position of the uppermost one of the stacked papers at all times.